Old Gas, New Gas

Just about a year ago, on January 14, 2005, the European Space
Agency's Huygens probe completed a one-way, 4-billion-kilometer
journey to Saturn's largest moon, Titan. The temperature there on
the surface is around -179 degrees, cold enough that methane is a
liquid. If you look at some of the startling images sent back
by Huygens you can see river-like channels that were probably carved
by streams of the stuff. Why methane? After nitrogen, it's the most
abundant component of Titan's atmosphere, and a heated sample of the
Titan soil released a puff of it.

So where does Titan's methane come from? People think it's that
serpentinization reaction mentioned earlier.